1. Field of the Invention
The present invention relates to an exhaust gas purifying system. More particularly, the present invention relates to an exhaust gas purifying system capable of effectively purifying a large amount of hydrocarbons (HC) discharged from a vehicle at a low temperature at time of starting up an engine.
2. Description of the Related Art
In order to purify exhaust gas from an internal combustion engine of an automobile or the like, a three-way catalyst that simultaneously performs oxidation of carbon monoxide (CO) and hydrocarbons (HC) and reduction of nitrogen oxides (NOx) has been widely used. However, at a low temperature at time of starting up the engine, the three-way catalyst is not activated because of the low temperature, and thus a large amount of cold HC discharged in this case cannot be purified.
Recent years, for the purpose of purifying such cold HC, a three-way catalyst added with a HC adsorbing function (hereinafter referred to as a HC-trap-catalyst) has been developed, which includes zeolite as a hydrocarbon adsorbent (HC adsorbent) and a purifying catalyst such as a three-way catalyst.
The HC-trap-catalyst contains the HC adsorbent and a material of the three-way catalyst. The HC-trap-catalyst temporarily adsorbs and holds cold HC discharged in a low temperature range at the time of starting up the engine, in which the three-way catalyst is not activated. Then, the HC-trap-catalyst gradually desorbs and even purifies the HC when the three-way catalyst is activated due to a temperature increase of the exhaust gas.
As the catalyst purifying the HC desorbed from the HC adsorbent, a catalyst obtained by mixing noble metal species such as rhodium (Rh), platinum (Pt) and palladium (Pd) in the same layer and a catalyst of a multilayer structure including Rh and Pd layers have been proposed. Japanese Patent Laid-Open Publication H2-56247 (published in 1990) discloses an exhaust gas purifying catalyst that includes a first layer mainly containing zeolite as a HC adsorbent and a second layer provided on the first layer. The second layer mainly contains noble metals such as Pt, Pd and Rh.
Exhaust gas purifying systems, each using the HC-trap-catalysts as described above, have been disclosed in Japanese Patent Laid-Open Publications H6-74019 (published in 1994), H7-144119 (published in 1995), H6-142457 (published in 1994), H5-59942 (published in 1993), H7-102957 (published in 1995), H7-96183 (published in 1995) and H11-81999 (published in 1999).
In the case of using the HC-trap-catalyst, the cold HC adsorbed to the HC adsorbent at the time of starting up the engine is often desorbed before an exhaust gas temperature reaches an activation temperature of the three-way-catalyst. While the three-way catalyst in the HC-trap-catalyst is not sufficiently activated, the desorbed HC are discharged in an unpurified state. Accordingly, in order to increase purification efficiency of the cold HC, it is desired to delay desorption of the HC.
In the HC-trap-catalyst having the structure in which the adsorbent layer and the three-way catalyst layer are laminated, in order to efficiently purify the desorbed HC in the three-way catalyst layer without inhibiting an adsorbing function of the adsorbent, studies have been conducted on a film thickness of the three-way catalyst layer. However, many types of HC components are contained in actual exhaust gas, and purification performance of the HC-trap-catalyst is varied depending also on the types of HC. Therefore, just by optimizing the film thickness of the three-way catalyst layer, reduction of the desorption of the adsorbed HC or the delay of the desorption thereof are insufficient, and it is difficult to sufficiently improve purifying characteristics for the cold HC.
Moreover, in order to improve the purifying efficiency for the cold HC, the following methods have been studied. In one method, the three-way catalyst is sufficiently activated by switching exhaust passages, and then the adsorbed HC are desorbed to be purified by the three-way catalyst. In another method, the three-way catalyst is activated early by an electric heater. In the other method, air is introduced from the outside to advance a start of activating the three-way catalyst. However, these methods are costly because of complex system constitutions, and in addition, cannot sufficiently raise the purification efficiency of the cold HC.
Furthermore, a method has been studied, in which a heat capacity of an exhaust pipe or the like upstream of the HC-trap-catalyst is increased, and thus a temperature increase of the HC-trap-catalyst is delayed, and a desorption rate of the adsorbed HC from the HC-trap-catalyst is reduced. However, the desorption rate of the adsorbed HC is greatly affected not only by the temperature but also an amount and a flow rate of the exhaust gas diffused in the HC-trap-catalyst, and therefore, a sufficient effect of reducing the desorption is not obtained. In addition, since the delay of the temperature increase delays the start of activating the three-way catalyst, the desorbed HC cannot be sufficiently purified.
An object of the present invention is to provide an exhaust gas purifying system that has a relatively simple constitution, can purify the cold HC efficiently, and is excellent in the purification performance for HC, CO and NOx from a cold range to a hot range.
In order to achieve the object, an exhaust gas purifying system according to an aspect of the present invention is a system for purifying gas exhausted from an engine, including a gas passage for gas exhausted from the engine, a first three-way catalyst arranged on the exhaust gas passage, a first HC-trap-catalyst arranged downstream of the first three-way catalyst on the gas passage, a second HC-trap-catalyst arranged downstream of the first HC-trap-catalyst on the gas passage, and a heat capacitor arranged between the first HC-trap-catalyst and the second HC-trap-catalyst on the gas passage. Here, each of the heat capacitor and the second HC-trap-catalyst has a honeycomb carrier, and a ratio Gt/Gh2 of a geometric surface area Gt of the honeycomb carrier of the heat capacitor to a geometric surface area Gh2 of the honeycomb carrier of the second HC-trap-catalyst is in a range of 1 to 5. Note that a geometric surface area G of the honeycomb carrier is a value obtained by dividing an inner surface area of the honeycomb carrier by a capacity of the honeycomb carrier.